A conventional hydraulic brake system for a vehicle provided with an anti-lock brake control is disclosed in, for example, the service manual "TOYOTA CORONA FF COUPE"(4-p.28.about.4-p.40) issued by the service department of TOYOTA AUTOMOTIVE CORPORATION on Aug. 4, 1987.
This hydraulic brake system includes a brake circuit for connecting the master cylinder to wheel brakes, a pressure control valve unit for reducing the hydraulic brake pressure in the wheel brake by discharging brake fluid from the wheel brake to a reservoir, and a plunger pump driven by an electric motor for returning brake fluid from the reservoir to the brake circuit. A pump chamber of the plunger pump is isolated from the brake circuit by closing the discharge valve and the suction valve of the plunger pump in a normal condition. The valve opening pressure of the discharge valve and the suction valve generally reaches several atmospheres to increase the discharge efficiency of the pump.
In recent years, when filling a hydraulic brake system with brake fluid, an evacuation method has been adopted in the assembly process of the automotive vehicle. After installing the hydraulic brake system in a vehicle, an apparatus evacuates the brake system through a master cylinder reservoir to pump the air out of the brake system. Then the apparatus supplies brake fluid to the brake system.
However, in the aforementioned conventional hydraulic brake system, since the pump chamber of the plunger pump is isolated from the brake circuit, the apparatus cannot evacuate air in the pump chamber.
A need exists, therefore, for a hydraulic brake system which is not susceptible of such a problem.